Ladies won't leave coop

I got my first chickens about a week ago-have never had chickens before. The flock is 8 older birds that someone gave me-6 hens, 2 roosters-these birds have been together for about 2 years. I kept them in their coop and pen for a week so that would learn they are home. I let them out of this afternoon into the yard so they could roam a little. I had planned on letting them have the run of the yard each day. They wouldn't leave their pen--is this normal? How can I encourage them to leave the pen?

Thanks so much. I have the two roosters because the entire flock was given to me as my starter flock. All 8 birds have been together for almost two years and it seems pretty stable. There is definitely a hierarchy, but it seems to work. I will continue to keep an eye on them and remove one or both if they get to be a problem. Originally, I hadn't planned on having any roosters, but it was all or none.